Method and apparatus for spin-echo-train MR imaging using prescribed signal evolutions

ABSTRACT

A magnetic resonance imaging “MRI” method and apparatus for lengthening the usable echo-train duration and reducing the power deposition for imaging is provided. The method explicitly considers the t1 and t2 relaxation times for the tissues of interest, and permits the desired image contrast to be incorporated into the tissue signal evolutions corresponding to the long echo train. The method provides a means to shorten image acquisition times and/or increase spatial resolution for widely-used spin-echo train magnetic resonance techniques, and enables high-field imaging within the safety guidelines established by the Food and Drug Administration for power deposition in human MRI.

Notice: More than one reissue application has been filed for the reissueof U.S. Pat. No. 7,164,268. The reissue applications are Ser. No.16/195,079 (the present application), application Ser. No. 14/708,875(now U.S. Pat. No. RE47,178), filed May 11, 2015, application Ser. No.14/053,190 (now U.S. Pat. No. RE45,725), filed Oct. 14, 2013, andapplication Ser. No. 12/354,471 (now U.S. Pat. No. RE44,644), filed Jan.15, 2009. The present application is a reissue continuation ofapplication Ser. No. 14/708,875.

RELATED APPLICATIONS

This application is a national stage reissue continuation of U.S. patentapplication Ser. No. 14/708,875, filed on May 11, 2015, which is areissue continuation of U.S. patent application Ser. No. 14/053,190,filed on Oct. 14, 2013 (now, U.S. Pat. No. RE45,725), which was areissue continuation of U.S. patent application Ser. No. 12/354,472,filed on Jan. 15, 2009 (now, U.S. Pat. No. RE44,644), which was areissue of U.S. Pat. No. 7,164,268, which issued from U.S. patentapplication Ser. No. 10/451,124, filed on Jun. 19, 2003 as a U.S.National Stage filing of International Application No. PCT/US01/50551,filed 21 Dec. 2001, which claims benefit priority under 35 U.S.C.Section 119(e) from U.S. Provisional Patent Application Ser. No.60/257,182, filed on Dec. 21, 2000, entitled “Spin-Echo-Train MR ImagingUsing Prescribed Signal Evolutions,” the entire disclosure disclosuresof which is hereby are incorporated by reference herein. The presentapplication is related to U.S. Pat. No. 5,245,282, filed on Jun. 28,1991, entitled “Three-dimensional Magnetic Resonance Imaging,” theentire disclosure of which is hereby incorporated by reference herein.

GOVERNMENT SUPPORT

Work described herein was supported by Federal Grant Number NS-35142,awarded by the National Institutes of Health. The United StatesGovernment possesses certain rights in and to this invention.

FIELD OF INVENTION

The present invention relates to a pulse sequence for use in operating amagnetic resonance imaging apparatus, and in particular for lengtheningthe usable echo-train duration and reducing the power deposition forspin-echo-train magnetic resonance imaging.

BACKGROUND OF INVENTION

Over the past twenty years, nuclear magnetic resonance imaging (MRI) hasdeveloped into an important modality for both clinical and basic-scienceimaging applications. A large portion of MRI techniques are based onspin-echo (SE) acquisitions because they provide a wide range of usefulimage contrast properties that highlight pathological changes and areresistant to image artifacts from a variety of sources such asradio-frequency or static-field inhomogeneities.

Spin-echo-based methods can be subdivided into two categories, includingthose that generate one spin echo for each desired image contrastfollowing each excitation radio-frequency (RF) pulse, and those thatgenerate more than one spin echo for each desired image contrastfollowing each excitation RF pulse. The first category includes, but isnot limited thereto, the techniques commonly referred to as“conventional SE” imaging. The second category includes, but is notlimited thereto, a method called “RARE” (See Hennig J., Nauerth A.,Friedburg H., “RARE Imaging: A Fast Imaging Method for Clinical MR”,Magn. Reson. Med. 1986, 3:823-833; and Mulkern R. V., Wong S. T. S.,Winalski C., Jolesz F. A., “Contrast Manipulation and ArtifactAssessment of 2D and 3D RARE Sequences”, Magn. Reson. Imaging 1990,8:557-566, of which are hereby incorporated by reference in theirentirety) and its derivatives, commonly referred to as “turbo-SE” or“fast-SE” imaging (See Melki P. S., Jolesz F. A., Mulkern R. V.,“Partial RF Echo Planar Imaging with the FALSE Method. I Experimentaland Theoretical Assessment of Artifact”, Magn. Reson. Med. 1992,26:328-341 and Jones K. M., Mulkern R. V., Schwartz R. B., Oshio K.,Barnes P. D., Jolesz F. A., “Fast Spin-Echo MR Imaging of the Brain andSpine: Current Concepts”, AJR 1992, 158:1313-1320, of which are herebyincorporated by reference in their entirety). For the purposes of thisdisclosure, we are primarily interested in the generalized form oftechniques in the second category; however the present invention isapplicable to the first category as well. The term “generalized” refersto the form of the spatial-encoding gradients that are applied followingany given refocusing RF pulse. For example, RARE imaging encodes oneline of spatial-frequency space (k-space) data following each refocusingRF pulse using a constant, frequency-encoding magnetic field gradient.In contrast, “GRASE” imaging (See Feinberg D. A., Oshio K. “GRASE(Gradient- And Spin-Echo) MR Imaging: A New Fast Clinical ImagingTechnique”, Radiology 1991, 181: 597-602; and Oshio K., Feinberg D. A.“GRASE (Gradient- And Spin-Echo) Imaging: A Novel Fast MRI Technique”,Magn. Reson. Med. 1991, 20:344-349, of which are hereby incorporated byreference in their entirety) encodes three or more lines of k-space datafollowing each refocusing RF pulse using an oscillating,frequency-encoding gradient waveform. This oscillating gradient waveformcollects one line of k-space data that includes the spin echo, and oneor more additional lines of k-space data before the spin echo and afterthe spin echo. One skilled in the art would appreciate that there existan infinite number of possibilities for spatially encoding the MR signalfollowing each refocusing RF pulse. For the purpose of this disclosure,we define the term “spin-echo-train” imaging to encompass all of thesepossibilities, including, but not limited thereto, RARE, turbo-SE,fast-SE and GRASE imaging, because the present invention deals with,among other things, the RF-pulse history during the echo train, not thedetails of the spatial encoding.

In general, one of the major goals of technique development for MRI hasbeen to increase the amount of k-space data sampled per unit time, underthe constraints of obtaining the desired image contrast and maintainingimage artifacts at a tolerable level. Increases in the data rate aretypically traded for a decrease in the image acquisition time and/or anincrease in the spatial resolution. In this respect, spin-echo-trainmethods have played an important role; fast-SE imaging is routinely andwidely used in clinical MRI.

For instance, the echo trains used in clinical fast-SE imaging generallyemploy high flip angles (>100°) for the refocusing RF pulses, and theirdurations are typically less than the T2 relaxation times of interestfor short effective echo times (e.g., T1 or proton-density weighting) orless than two to three times these T2 values for long effective echotimes (e.g., T2 weighting or “FLAIR”; see Hajnal J. V., Bryant D. J.,Kasuboski L., Pattany P. M., De Coene B., Lewis P. D., Pennock J. M.,Oatridge A., Young I. R., Bydder G. M., “Use of Fluid AttenuatedInversion Recovery (FLAIR) Pulse Sequences in MRI of the Brain”, J.Comput. Assist. Tomogr. 1992, 16:841-844, of which is herebyincorporated by reference in its entirety). For example, consideringbrain imaging at 1.5 Tesla, these limits translate to echo-traindurations of <100 ms and <300 ms for short and long effective echotimes, respectively. When high flip angles are used for the refocusingRF pulses, echo-train durations that are longer than these limits cansubstantially degrade image contrast and introduce artifacts such asblurring (See Mulkern et al.; Melki et al.; Constable R. T., Gore J. C.,“The Loss of Small Objects in Variable TE Imaging: Implications for FSE,RARE and EPI”, Magn. Reson. Med. 1992, 28:9-24; and Ortendahl D. A.,Kaufman L., Kramer D. M., “Analysis of Hybrid Imaging Techniques”, Magn.Reson. Med. 1992, 26:155-173, of which are hereby incorporated byreference in their entirety).

Nonetheless, if it were possible to substantially lengthen echo-traindurations beyond these limits, while achieving the desired imagecontrast and limiting artifacts, it would represent a useful and widelyapplicable advance.

Preliminary studies with the goal of lengthening the echo-train durationin spin-echo-train-based acquisitions have been performed by otherresearchers. Over a decade ago, Hennig (See Hennig J., “MultiechoImaging Sequences with Low Refocusing Flip Angles”, J. Magn. Reson.1988, 78:397-407, of which is hereby incorporated by reference in itsentirety) proposed the use of constant, low-flip-angle refocusing RFpulses to introduce a T1 dependence to the evolution of the echo trainand thereby lengthen its usable duration. More recently, this conceptwas extended by Alsop, who derived variable flip-angle series based onthe “pseudosteady-state” condition of a constant signal level when T1and T2 relaxation are neglected (See Alsop D. C., “The Sensitivity ofLow Flip Angle RARE Imaging”, Magn. Reson. Med. 1997; 37:176-184, ofwhich is hereby incorporated by reference in its entirety). Alsop alsofound that the echo-train performance was improved by using a signalevolution that decreased for the first few echoes and was then constant,instead of being constant for the complete echo train. Using theseevolutions, artifact-free human brain images with T2-weighting wereacquired by Alsop. An 80-echo train with a duration of 400 ms andasymptotic flip angles ranging from 17° to 90° were used.

Turning to the present invention, a method and related apparatus isprovided for lengthening the usable echo-train duration forspin-echo-train imaging substantially beyond that achievable with theconstant, low-flip-angle or pseudosteady-state approaches. The presentinvention method and apparatus explicitly consider the T1 and T2relaxation times for the tissues of interest and thereby permit thedesired image contrast to be incorporated into the tissue signalevolutions corresponding to the long echo train. Given the considerablerole that spin-echo-train methods already play in MR imaging, thepresent invention methodology will be of significant importance.

SUMMARY OF THE INVENTION

This present invention comprises the methodology, related apparatus, andcomputer useable medium (readable media) for using a series ofrefocusing RF pulses with variable flip angles and, optionally, variablephase angles, in a spin-echo-train MRI pulse sequence wherein theflip-angle series is specifically designed to achieve a prescribedsignal evolution during the echo train for selected T1 and T2 relaxationtimes. By employing such a series of refocusing RF pulses, the usableduration of the echo train can be extended substantially beyond thatobtainable with conventional methods. This increase in the echo-trainduration can be used to decrease the image acquisition time and/orincrease the spatial resolution.

In one aspect, the present invention features a method for generating apulse sequence for operating a magnetic resonance imaging apparatus forimaging an object, the method comprising:

-   a) providing contrast-preparation, the contrast-preparation    comprising generating at least one of at least one radio-frequency    pulse, at least one magnetic-field gradient pulse, and at least one    time delay, whereby the contrast preparation encodes the    magnetization with at least one desired image contrast;-   b) calculating flip angles and phases of refocusing radio-frequency    pulses that are applied in a data-acquisition step, wherein the    calculation provides desired prescribed signal evolution and desired    overall signal level, the calculation comprises:    -   i) selecting values of T1 and T2 relaxation times and selecting        proton density;    -   ii) selecting a prescribed time course of the amplitudes and        phases of the radio-frequency magnetic resonance signals that        are generated by the refocusing radio-frequency pulses; and    -   ii) selecting characteristics of the contrast-preparation step,        the data-acquisition step and a magnetization-recovery step,        with the exception of the flip angles and phases of the        refocusing radio-frequency pulses that are to be calculated; and-   c) providing the data acquisition step based on a spin echo train    acquisition, the data-acquisition step comprises:    -   i) an excitation radio-frequency pulse having a flip angle and        phase;    -   ii) at least two refocusing radio-frequency pulses, each having        a flip angle and phase as determined by the calculation step;        and    -   iii) magnetic-field gradient pulses that encode spatial        information into at least one of the radio-frequency magnetic        resonance signals that follow at least one of the refocusing        radio-frequency pulses;-   d) providing magnetization-recovery, the magnetization-recovery    comprises a time delay to allow magnetization to relax; and-   e) repeating steps (a) through (d) until a predetermined extent of    spatial frequency space has been sampled.

In a second aspect, the present invention features a magnetic resonanceimaging apparatus for generating a pulse sequence for operating theapparatus for imaging an object, the apparatus comprising a main magnetsystem for generating a steady magnetic field; a gradient magnet systemfor generating temporary gradient magnetic fields; a radio-frequencytransmitter system for generating radio-frequency pulses; aradio-frequency receiver system for receiving magnetic resonancesignals; a reconstruction unit for reconstructing an image of the objectfrom the received magnetic resonance signals; and a control unit forgenerating signals controlling the gradient magnet system, theradio-frequency transmitter system, the radio-frequency receiver system,and the reconstruction unit, wherein the control unit generates signalscausing:

-   a) providing contrast-preparation, the contrast-preparation    comprising generating at least one of at least one radio-frequency    pulse, at least one magnetic-field gradient pulse, and at least one    time delay, whereby the contrast preparation encodes the    magnetization with at least one desired image contrast;-   b) calculating flip angles and phases of refocusing radio-frequency    pulses that are applied in a data-acquisition step, wherein the    calculation provides desired prescribed signal evolution and desired    overall signal level, the calculation comprises:    -   i) selecting values of T1 and T2 relaxation times and selecting        proton density;    -   ii) selecting a prescribed time course of the amplitudes and        phases of the radio-frequency magnetic resonance signals that        are generated by the refocusing radio-frequency pulses; and    -   ii) selecting characteristics of the contrast-preparation step,        the data-acquisition step and a magnetization-recovery step,        with the exception of the flip angles and phases of the        refocusing radio-frequency pulses that are to be calculated; and-   c) providing the data acquisition step based on a spin echo train    acquisition, the data-acquisition step comprises:    -   i) an excitation radio-frequency pulse having a flip angle and        phase,    -   ii) at least two refocusing radio-frequency pulses, each having        a flip angle and phase as determined by the calculation step,        and    -   iii) magnetic-field gradient pulses that encode spatial        information into at least one of the radio-frequency magnetic        resonance signals that follow at least one of the refocusing        radio-frequency pulses;-   d) providing magnetization-recovery, the magnetization-recovery    comprises a time delay to allow magnetization to relax; and-   e) repeating steps (a) through (d) until a predetermined extent of    spatial frequency space has been sampled.

In a third aspect, the present invention features a computer readablemedia carrying encoded program instructions for causing a programmablemagnetic resonance imaging apparatus to perform the method discussedabove in the first aspect of the invention. Similarly, the inventionfeatures a computer program product comprising a computer useable mediumhaving computer program logic for enabling at least one processor in amagnetic resonance imaging apparatus to generate a pulse sequence, thecomputer program logic comprising the method discussed above in thefirst aspect of the invention.

Because the flip angles for the refocusing RF pulses that are derivedwith this method are typically much less than 180° for a substantialportion of the total number of RF pulses, the power deposition is muchless than that corresponding to 180° RF pulses, which are commonly usedin conventional spin-echo-train pulse sequences. This feature isparticularly important for high field MRI (>1.5 Tesla), wherein powerdeposition is a critical pulse-sequence design factor for humanapplications. The present invention permits long, closely-spaced echotrains to be used for high-field imaging that would not otherwise meetthe safety guidelines established by the Food and Drug Administrationfor power deposition in human MRI.

Another potentially useful feature of the present invention is that, forspecific forms of the encoding-gradient waveforms, signals from movingor flowing materials are strongly attenuated, even when the velocitiesare relatively low. A specific example of this behavior is theattenuation of the signal from cerebrospinal fluid (CSF) surrounding thecervical spinal cord due to its oscillatory motion, which can be used togenerate CSF-suppressed T2-weighted MR images of the spinal cord withoutrequiring inversion-nulling of the CSF signal. Studies have indicatedthat the full range of clinically-relevant cord lesions may not beadequately detected using inversion-nulling of the CSF signal (i.e.,FLAIR) (See Hittmair K., Mallek R., Prayer D., Schindler E. G.,Kollegger H., “Spinal Cord Lesions in Patients with Multiple Sclerosis:Comparison of MR Pulse Sequences”, AJNR 1996, 17:1555-1565; and KeiperM. D., Grossman R. I., Brunson J. C., Schnall M. D., “The LowSensitivity of Fluid-Attenuated Inversion-Recovery MR in the Detectionof Multiple Sclerosis of the Spinal Cord”, AJNR 1997, 18:1035-1039, ofwhich are hereby incorporated by reference in their entirety).

These and other objects, along with advantages and features of theinvention disclosed herein, will be made more apparent from thedescription, drawings and claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention, as well as the invention itself, will be more fullyunderstood from the following description of preferred embodiments, whenread together with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a general spin-echo-train MRIpulse sequence. This is an exemplary type of MRI pulse sequence to whichthe invention applies. The present invention method can be applied tovarious types of pulse sequences.

FIG. 2 shows an example of a prescribed signal evolution that can beused to generate T2-weighted MR images of the brain or spine.

FIG. 3 shows the variable-flip-angle series corresponding to FIG. 2 thatwas derived using the present invention methods as described herein.

FIGS. 4-6 show example MR images obtained using the variable-flip-angleseries of FIG. 3 in a “turbo-SE” type spin-echo-train pulse sequence;collectively, FIGS. 4-6 provide examples of the potential utility of thepresent invention. In particular, showing brain images obtained at 1.5Tesla, FIGS. 4(A) and 4(B)-4(C) compare T2-weighted two-dimensional andthree-dimensional SE images, respectively. Further, showing brain imagesobtained at 3 Tesla, FIGS. 5(A)-5(C) show T2-weighted sagittal, coronal,and axial images, respectively, reconstructed from the samethree-dimensional acquisition. Finally, FIG. 6 shows a sagittal image ofthe cervical spinal cord obtained at 1.5 Tesla.

FIG. 7 illustrates a simplified exemplary embodiment of a MRI apparatusfor practicing the present invention. The present invention method canbe applied to various commercially available MRI apparatuses.

FIG. 8 is an exemplary flowchart for a simplified preferredimplementation of the methods of the present invention.

FIGS. 9A-9B is an exemplary flowchart for a simplified preferredimplementation of the calculation methods of the present invention, step200.

FIG. 10 is an exemplary flowchart for a simplified preferredimplementation of the contrast-preparation methods of the presentinvention.

FIG. 11 is an exemplary flowchart for a simplified preferredimplementation of the data-acquisition methods of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, first presented is an exemplary embodiment of a MRapparatus for practicing the MR methods of the present invention forimaging an object, moving or stationary. Following are descriptions ofpreferred and alternative embodiments of the methods of the presentinvention, including their exemplary implementation as computerhardware, firmware, and/or software.

An Exemplary MR-Apparatus of the Present Invention

FIG. 7 illustrates a simplified schematic of a MR apparatus 1 forpracticing the present invention. The MR apparatus 1 includes a mainmagnet system 2 for generating a steady magnetic field in an examinationzone(s) of the MR apparatus. The z-direction of the coordinate systemillustrated corresponds to the direction of the steady magnetic fieldgenerated by the magnet system 2.

The MR apparatus also includes a gradient magnet system 3 for generatingtemporary magnetic fields G_(x), G_(y) and G_(z) directed in thez-direction but having gradients in the x, y or z directions,respectively. With this magnetic gradient system, magnetic-fieldgradients can also be generated that do not have directions coincidingwith the main directions of the above coordinate system, but that can beinclined thereto, as is known in the art. Accordingly, the presentinvention is not limited to directions fixed with respect to the MRapparatus. In this application, for ease of description, the directionsx, y and z (and the gradients along these directions) are used for theread direction, the phase-encode direction and slice-selection direction(or second phase-encode direction for 3D imaging), respectively.

Also, while traditional commercial methods provide linear gradients inthe x, y, or z directions it is also possible not to utilize all threeof these linear gradients. For example, rather than using a linear zgradient, one skilled in the art can use a z-squared dependence or someother spatial dependence to provide desired results.

The magnet systems 2 and 3 enclose an examination zone(s) which is largeenough to accommodate a part of an object 7 to be examined, for examplea part of a human patient. A power supply means 4 feed the gradientmagnet system 3.

The MR apparatus also includes an RF transmitter system including RFtransmitter coil 5, which generates RF pulses in the examination zoneand is connected via transmitter/receiver circuit 9 to a RF source andmodulator 6. The RF transmitter coil 5 is arranged around the part ofbody 7 in the examination zone. The MR apparatus also comprises an RFreceiver system including an RF receiver coil which is connected viatransmitter/receiver circuit 9 to signal amplification and demodulationunit 10. The receiver coil and the RF transmitter coil 5 may be one andthe same coil. The MR apparatus also includes an amplification anddemodulation unit 10, which, after excitation of nuclear spins in a partof the body placed within the examination space by RF pulses, afterencoding by the magnetic-field gradients and after reception of theresulting MR signals by the receiver coil, derives sampled phases andamplitudes from the received MR signals. An image reconstruction unit 12processes the received MR imaging signals to, inter alia, reconstruct animage by methods well-known in the art, such as by Fouriertransformation. It should be appreciated by one skilled in the art thatvarious reconstruction methods may be employed besides the FourierTransform (FT) depending on factors such as the type of signal beinganalyzed, the available processing capability, etc. For example, but notlimited thereto, the present invention may employ Short-Time FT (STFT),Discrete Cosine Transforms (DCT), or wavelet transforms (WT). By meansof an image processing unit 13, the reconstructed image is displayed,for example, on monitor 14. Further, the image reconstruction unit canoptionally process MR navigator signals to determine the displacement ofa portion of the patient.

The MR apparatus also includes a control unit 11 that generates signalsfor controlling the RF transmitter and receiver systems by means of amodulator 6, the gradient magnetic field system by means of the powersupply means 4, an image reconstruction unit 12 and an image processingunit 13. In a preferred embodiment, the control unit 11 (and othercontrol elements in the MR apparatus) are implemented with programmableelements, such as one or more programmable signal processors ormicroprocessors, communicating over busses with supporting RAM, ROM,EPROM, EEPROM, analog signal interfaces, control interfaces, interfaceto computer-readable media and so forth. These programmable elements arecommanded by software or firmware modules loaded into RAM, EPROM, EEPROMor ROM, written according to well-known methods to perform the real-timeprocessing required herein, and loaded from computer-readable media (orcomputer useable medium), such as magnetic disks or tapes, or opticaldisks, or network interconnections, removable storage drives, or soforth. The present invention may be implemented using hardware, softwareor a combination thereof and may be implemented in one or more computersystems or processing systems, such as personal digit assistants (PDAs),for various applications, e.g., remote care and portable care practices.

In a less preferred embodiment, the control unit that directs a MRapparatus for practicing the present invention can be implemented withdedicated electronic components in fixed circuit arrangements. In thiscase, these dedicated components are arranged to carry out the methoddescribed above. For example, the invention is implemented primarily inhardware using, for example, hardware components such as applicationspecific integrated circuits(ASICs). Implementation of the hardwarestate machine to perform the functions described herein will be apparentto persons skilled in the relevant art(s).

In particular, the control unit commanded by its loaded software causesthe generation of MR signals by controlling the application of MR pulsesequences, which comprise RF-pulses, time delays and temporarymagnetic-field gradient pulses. These pulse sequences are generatedaccording to the methods of the present invention as subsequentlydescribed, and generally include 2D and 3D imaging pulse sequences andoptionally navigator pulse sequences for determining the displacement ofthe patient or material.

Furthermore, according to alternate embodiments of the presentinvention, the MR apparatus also optionally includes various other units(not illustrated) from which the state of motion of the part of thepatient being imaged can be measured. These can include sensors directlyindicating the instantaneous state of motion of the part of the patientbeing imaged, such as a chest belt for directly indicating chestdisplacement during respiration, or MR-active micro-coils whose positioncan be tracked, or optical means, or ultrasound means, or so forth.These units can also include sensors indirectly indicating theinstantaneous state of motion of the part of the patient being imaged.For example, electrocardiogram and peripheral pulse sensors measure thetemporal progress of the cardiac cycle, and permit inference of theactual state of motion of the heart from knowledge of cardiacdisplacements associated with each phase of the cardiac cycle. Whenthese sensors are present to measure the state of motion, the controlunit need not generate navigator pulse sequences.

Moreover, the control unit 11 may also include a communicationsinterface 24. The communications interface 24 allows software and datato be transferred between and among, via communication path (i.e.channel) 28, the control unit 11, reconstruction unit 12, imageprocessing unit 13, and monitor 14 and external devices. Examples of thecommunications interface 24 may include a modem, a network interface(such as an Ethernet card), a communications port, a PCMCIA slot andcard, etc. Software and data transferred via communications interface 24are in the form of signals which may be electronic, electromagnetic,optical or other signals capable of being received by communicationsinterface 24. The signals are provided to communications interface 24via the communications path (i.e., channel) 26. The channel 26 carriessignals and may be implemented using wire or cable, fiber optics, aphone line, a cellular phone link, a RF link, IR link and othercommunications channels.

The preferred embodiments of the present invention may be implemented assoftware/firmware/hardware with various MR apparatuses, and methods, asone skilled in the art would appreciate. Other exemplary apparatuses andmethods, but not limited thereto, are disclosed in the following U.S.patents, of which are hereby incorporated by reference in their entiretyherein: U.S. Pat. No. 6,230,039 B1—Staber et. al.; U.S. Pat. No.5,749,834—Hushek; and U.S. Pat. No. 5,656,776—Kanazawa.

The Methods of the Present Invention

Turning now to FIG. 1, pertaining to the general methods of thisinvention, first in a preferred embodiment, the present inventionapplies to magnetic resonance imaging MRI) using a “spin-echo-train” MRIpulse sequence, which is a pulse sequence that generates more than onespin echo for each desired image contrast following each excitation RFpulse. Any form of the applied spatial-encoding gradient waveforms,variations in the spacing between refocusing RF pulses, and/or anycombination of non-selective, spatially-selective, andspectrally-selective RF pulses are applicable to the present inventionmethods as long as their effects on the magnetization are appropriatelyconsidered in the derivation of the variable-flip-angle series. Acontrast preparation phase, such as an inversion RF pulse followed by atime delay, may precede the acquisition phase of the pulse sequence.

Still referring to FIG. 1, there is shown a schematic representation ofa general spin-echo-train MRI pulse sequence. This is an exemplary typeof MRI pulse sequence to which the invention applies. The representationis of a general spin-echo-train MRI pulse sequence showing theexcitation RF pulse (α) and the first three (β₁, β₂, β₃) of n refocusingRF pulses, where n>1. The RF pulse waveforms are drawn as rectangularfor simplicity, but they may be amplitude and/or phase modulated asappropriate for the desired application. The echo spacing (ESP) may befixed or may vary between echoes. The contrast preparation module 20denotes the optional use of additional RF pulses, gradient pulses and/ortime delays (e.g., an inversion pulse followed by a time delay) topermit additional control over the image contrast. The boxes on theG_(select) axis, referenced as 31, 32, 33 and 34, symbolically denotethe optional use of magnetic-field gradient waveforms for spatial and/orspatial-spectral selection. The boxes on the G_(encode) axis, referencedas 41, 42, and 43, symbolically denote the magnetic-field gradientwaveforms used for spatial encoding. The contrast preparation (if any)and the echo train are repeated as necessary to collect the desiredk-space data. The timing parameters and the number of echoes may varybetween repetitions.

For a spin-echo-train pulse sequence, an object of the present inventionis to derive a series of refocusing RF pulses with variable flip angles,and, optionally, variable phase angles, that yields a specificallyprescribed signal evolution during the echo train for selected T1 and T2relaxation times. To achieve this, a mathematical model of the pulsesequence, incorporating the specific timing, gradient and RF parametersof choice, is used to calculate the signal evolution during the echotrain. This model would typically be implemented in the form of acomputer program that is based on the established mathematical equationsthat describe the behavior of the magnetization during a pulse sequence.See Haacke E. M., Brown R. W., Thompson M. R., Venkatesan R., “MagneticResonance Imaging: Physical Principles and Sequence Design”, John Wiley& Sons, New York, 1999, of which is hereby incorporated by reference inits entirety. Other exemplary spin-echo-train MR imaging methods aredisclosed in the following U.S. patents, of which are herebyincorporated by reference in their entirety herein: U.S. Pat. No.5,680,045—Feinberg; U.S. Pat. No. 5,612,619—Feinberg; U.S. Pat. No.5,541,511—Henning; U.S. Pat. No. 5,315,249—Le Roux et al.; U.S. Pat. No.5,270,654—Feinberg et al.; U.S. Pat. No. 4,901,020—Ladebeck et al. andU.S. Pat. No. 4,818,940—Henning et al.

Given such a computer-based calculation tool, the process for derivingthis flip-angle series can be generally summarized in the following foursteps (steps I-IV) briefly discussed below. Firstly, STEP I, the pulsesequence timing parameters (e.g., repetition time, echo spacing(s),other time delays), the pulse sequence magnetic-field gradientconfiguration, the desired shape of the prescribed signal evolutionduring the echo train, the T1 and T2 relaxation parameters and theproton density for the “target” tissue, and a target signal intensityare chosen. The signal evolution may assume any physically-realizableshape. Some examples, but not limited thereto, include: a constant; alinear decay; an exponential decay; a linear or exponential decay forthe initial portion and a constant for the remainder; and a linear orexponential decay for the initial portion, a constant for the secondportion and a linear or exponential decay for the remainder. The T1, T2and proton density for the target tissue may equal those for a specificbiological tissue (e.g., brain gray matter) or material, or they may bearbitrarily chosen. The target signal intensity is the desired signalintensity corresponding to a specific echo in the echo train (e.g., thefirst or the middle echo).

Secondly, STEP II, the flip angle β_(i) (see FIG. 1) which yields thedesired signal intensity for the i^(th) echo interval is determined,where i ranges from 1 to the number of refocusing RF pulses in the echotrain. This flip angle can be calculated using any appropriate methodsuch as a “brute-force” search or interval bisection and interpolation.See Forsythe G. E., Malcolm M. A., Moler C. B., “Computer Methods forMathematical Computations”, Prentice-Hall, Englewood Cliffs, 1977, ofwhich is hereby incorporated by reference in its entirety.

Thirdly, STEP III, the pulse number i is incremented and the second stepis repeated until all flip angles for a given echo train are calculated.If, for any value of i, the desired signal intensity for the i^(th) echointerval cannot be achieved, the target signal intensity is reduced andthe calculation process is restarted.

Fourthly, STEP IV, if the pulse sequence under consideration requiresmore than one repetition of the echo train to acquire the desiredk-space data, the second and third steps are repeated as necessary untila steady state of the magnetization is reached.

After a given series of variable flip angles are derived, the targetsignal intensity can be incremented until the maximum value for whichthe prescribed signal-evolution shape can be realized is reached, thusallowing determination of the maximum signal and/or contrast values thatcan be obtained for a specific pulse sequence configuration and signalevolution.

Next, exemplary hardware, firmware and software implementations of themethods of the present invention are discussed.

FIG. 8 illustrates a preferred method for practicing the invention asimplemented by, for example, software loaded into the control unit ofthe MR apparatus. Once the process starts and initializes, at step 200,the contrast-preparation is provided by generating at least one of a RFpulse, magnetic-field gradient pulse, and/or time delay. Thecontrast-preparation encodes the magnetization with at least one desiredimage contrast.

During step 300, flip angles and phases are calculated for refocusing RFpulses that are applied in subsequent data-acquisition steps so as toyield—for selected values of T1 and T2 relaxation times and protondensity—a prescribed time course for the amplitudes and phases of the RFmagnetic resonance signals that are generated by the refocusing RFpulses.

During step 400, data-acquisition is achieved based on an echo-trainacquisition, comprising the following: i) an excitation RF pulse havinga flip angle and phase; ii) at least two refocusing RF pulses, eachhaving a flip angle and phase as determined by the calculation step; andiii) magnetic-field gradient pulses that encode spatial information intoat least one of the RF magnetic resonance signals that follow at leastone of the at least two refocusing RF pulses.

Also, in step 500, magnetization-recovery is provided whereby themagnetization-recovery comprises a time delay to allow magnetization torelax. Finally, as illustrated by step 550, the aforementioned method isrepeated until a predetermined extent of spatial-frequency space hasbeen sampled.

It is important to appreciate that the various steps discussed hereinneed not be performed in the illustrated order, and in fact it may bepreferred to perform the steps, at least in part, simultaneously or omitsome of the illustrated steps, at least in part.

Next, turning to FIGS. 9A-9B, an exemplary method for the calculatingstep 300 is provided. In step 310, T1, T2, and proton density arechosen, and in step 320, a desired prescribed signal evolution whichdescribes the time course of the signal amplitudes and phases, is alsochosen. Turning to steps 330 and 340, the characteristics of thecontrast-preparation and data-acquisition, respectively, are chosen(additional details shall be discussed with FIGS. 10-11). Provided instep 350, the characteristics of the magnetization-recovery period arechosen. Accordingly, at step 360, the process of calculating theindividual flip angles starts and is initialized with thermalequilibrium magnetization. At step 370, the magnetization, M₁, iscalculated that exists immediately after the contrast-preparation andexcitation RF pulse are applied. At step 380, starting with M₁ which isthe input magnetization for the first refocusing RF pulse, the flipangle and phase are calculated for the current refocusing RF pulse thatyields the desired corresponding signal amplitude and phase, and thisprocess is repeated until the last refocusing RF pulse is achieved, step385. If the last pulse is achieved, then step 390 permits relaxationduring the magnetization-recovery period. In steps 392 and 394, theprocess checks for a single-shot pulse sequence method and whethersteady state has been achieved. This accounts for the effects ofmultiple applications of the contrast-preparation, data-acquisition andmagnetization-recovery steps if required to sample the desired extent ofspatial-frequency space. As used herein, the “steady state” ofmagnetization is a state created by certain fast (with repeat timesshort compared to relaxation times) imaging pulse sequences during whichboth the longitudinal and the transverse components of the nuclearmagnetization exhibit a steady temporal state. Once steady state issatisfied the process checks whether the target signal has beenachieved, step 396, so as to increment across the overall signal level.

Turning to the exemplary contrast-preparation process as shown in FIG.10, at step 331, the following are chosen: flip angle, phase, waveformand the time of application of any RF pulses. In step 332, the followingare chosen: strength, duration, time-dependence, axis and time ofapplication of any magnetic-field gradient pulses. Moreover, in step333, the following are chosen: duration and time of application of anytime delays.

Turning to the exemplary data-acquisition process as shown in FIG. 11,at step 341, the following are chosen: the flip angle and phase of theexcitation RF pulse. Also, at steps 342, 343, and 344, the times betweenall RF pulses; number of refocusing RF pulses; and configuration ofspatial-encoding magnetic-field gradient pulses are chosen,respectively.

EXAMPLES

Specific implementations of the present invention methodology are usefulto illustrate its nature. These examples are non-limiting and areoffered as exemplary only. For this purpose, set forth herein areexperimental studies in which the present invention method was used togenerate variable-flip-angle series for three-dimensional (3D)T2-weighted MR imaging of the human brain and cervical spine using a“turbo-SE” type (i.e., RARE-as set forth in Henning et al., Magn. Res.Med. 1986, 3:823-833) spin-echo-train pulse sequence, of which is herebyincorporated by reference in its entirety. Brain studies were performedat 1.5 Tesla and 3 Tesla; spine studies were performed at 1.5 Tesla. MRimages were obtained using a 1.5-Tesla commercial whole-body imager(MAGNETOM SYMPHONY, Siemens Medical Systems, Iselin, N.J.) or a 3-Teslacommercial whole-body imager (MAGNETOM ALLEGRA, Siemens Medical Systems,Iselin, N.J.). The standard head RF coil supplied with the imager wasused. Informed consent was obtained from all subjects prior to imaging.

Turning to FIGS. 2-3, FIG. 2 shows an example of a prescribed signalevolution for gray matter that can be used to generate T2-weighted MRimages of the brain. The evolution consists of the following:exponential decay during the first 20 echoes with decay constant of 114ms, constant for 66 echoes, and exponential decay during the remainingechoes with decay constant of 189 ms. FIG. 3 shows the correspondingvariable-flip-angle series that was derived using the present inventionmethods as described herein. Using an interactive computer-based(Ultra-60 workstation; Sun Microsystems, Inc.) theoretical model, andthe prescribed signal evolution for brain gray matter, at 1.5 Tesla (seeFIG. 2), the four-step process described above was used to derive thecorresponding variable-flip-angle series depicted in FIG. 3. Thepulse-sequence parameters included an echo train length of 160, an echospacing of 4.1 ms (fixed), a total echo-train duration of 656 ms, arepetition time of 2750 ms and an effective echo time of 328 ms.

Example No. 1

FIGS. 4B-4F show an example of MR brain images obtained at 1.5 Teslausing the variable-flip-angle series of FIG. 3 in a “turbo-SE” typespin-echo-train pulse sequence; collectively. In particular, theT2-weighted two-dimensional and three-dimensional SE images of FIGS.4(A) and 4(B)-4(C), respectively, were obtained from a 59 year oldvolunteer for demonstrating age-related non-specific white-matterlesions. As can be observed, arrows mark several of these lesions. Theadjacent 1-mm thick 3D images, as shown in FIGS. 4B-4D, correspond tothe single 3-mm thick 2D image in FIG. 4A. In the 3D images, thephase-encoding direction corresponding to the 160-echo train isleft-to-right in FIGS. 4B-4D and 4F. No image artifacts secondary tothis very long spin-echo train are apparent. Pulse sequence parametersfor the 10 minute 3D acquisition included the following: repetitiontime/effective echo time, 2750/328 ms; matrix, 256×160×216; field ofview, 25.6×16.0×21.6 cm; voxel size, 1.0×1.0×1.0 mm; echo spacing, 4.1ms; echo train length, 160; and full-Fourier acquisition. Pulse sequenceparameters for the 14.8 minute 2D acquisition included the following:repetition time/first echo time/second echo time, 2750/20/80 ms; matrix,256×160; field of view, 25.6×16.0 cm; section thickness, 3.0 mm; numberof sections, 54; full-Fourier acquisition; first-order flowcompensation; and reduced bandwidth on second echo.

In summary, using the variable-flip-angle series of FIG. 3, theT2-weighted 3D images were obtained at 1.5 Tesla from the brain of ahealthy volunteer, and were compared to images from a 2D conventional-SEpulse sequence (see FIG. 4). The images in FIG. 4 exhibit two importantfeatures: (1) the very long spin-echo-train images (FIGS. 4B-4F) displayhigh contrast between the age-related lesions in the brain of thisvolunteer and surrounding normal appearing white matter, indicating thatthis echo train shall provide clinically useful contrast characteristicsthat appear very similar to those for conventional T2-weighed SE images(FIG. 4A); and (2) the thin 1-mm sections provide an improved definitionof lesion location and extent; the lesions seen in the 2D image appear,to varying degrees, in three adjacent 1-mm sections. Furthermore, theoverall image quality for the very long spin-echo-train andconventional-SE images is similar, despite the much thinner sections ofthe former.

Finally, referring to FIGS. 4E-4F, such figures depict the largestlesion in sagittal and coronal orientations, respectively, thisdemonstrates the capability of the 3D acquisition to providehigh-quality images in arbitrary orientations.

Example No. 2

Next, referring to FIGS. 5A-5C, using the same pulse-sequence parametersas described above in FIGS. 2-4, T2-weighted images were also obtainedat 3 Tesla from the brain of a healthy volunteer. The three 1-mm thickimages were all reconstructed from the same 3D acquisition. These imagesappeared similar to those obtained at 1.5 Tesla, but exhibited highersignal-to-noise ratios. Of particular importance, the partial-body andlocal values for the specific absorption rate (SAR) were 1.29 W/kg and3.16 W/kg, respectively, compared to the FDA limits for partial-body andlocal SAR of 3.0 and 8.0 W/kg, respectively. The SAR values at 3 Teslawere much less than the FDA limits, indicating that there remainssubstantial latitude in the pulse-sequence design from the perspectiveof power deposition, including the possibility for even more refocusingRF pulses per excitation. Thus, according to the present invention,although the use of spin-echo-train methods has been restricted at highfields, such as 3 Tesla, due to power deposition limits, very longspin-echo trains based on prescribed signal evolutions permithigh-quality brain images to be acquired at 3 Tesla with powerdeposition well below the FDA limits.

Example No. 3

Referring to FIG. 6, as the final example, FIG. 6 shows a T2-weightedsagittal image of the cervical spinal cord obtained at 1.5 Tesla from ahealthy volunteer, again using a 160-echo train. The quality ofcervical-spine images from T2-weighted MRI techniques is oftencompromised by artifacts arising from the pulsatile motion of the CSFsurrounding the cord. One potential solution to this problem is to useFLAIR imaging. See Hajnal et al. While this technique can completelysuppress the signal from CSF, there remains some concern about itsability to depict the full range of clinically-relevant lesions. SeeHittmair et al. and Keiper et al. As illustrated in FIG. 6, analternative is to use a T2-weighted technique with a very long spin-echotrain based on a prescribed signal evolution, as provided by the presentinvention. The signal from CSF is uniformly suppressed withoutgenerating motion artifacts. The combination of the long echo train andthe relatively low flip angles of the refocusing RF pulses results insuppression of even slowly moving fluid. The “dark-CSF” image in FIG. 6differs from a FLAIR image, among other things, in an important way.With FLAIR, the CSF is suppressed based on its long TI. Hence, thesignals from any other tissues with relatively long T1s will bediminished. This is one potential explanation for the problems indepicting certain lesions with FLAIR—these lesions may have long T1components. In contrast, turning to the present invention, the CSFsuppressed in FIG. 6 is solely due to its motion; long T1 lesions in thecord will be unaffected.

An advantage of the present invention is that it provides a method,apparatus, and computer useable medium (readable media) to extend theusable duration of the echo train in magnetic resonance imaging pulsesequences such as RARE, turbo-spin-echo, fast-spin-echo or GRASE,substantially beyond that obtainable with conventional methods. Thisincrease in the echo-train duration can be used to decrease the imageacquisition time and/or increase the spatial resolution. The powerdeposition achieved with this technique is much less than that forconventional spin-echo-train pulse sequences, and thus the inventionshall be especially useful, among other things, for human imagingapplications at high magnetic field strengths.

Another advantage of the present invention is that it improves theimaging of various objects and zones, including the brain. The presentinvention is also applicable to other regions of the body such as thespinal cord or joints. In particular, the present invention enableshigh-resolution 3D imaging of the brain with clinically-reasonableacquisition times, which is useful for quantitative imaging ofdisseminated diseases such as multiple sclerosis. For these diseases,high-resolution 3D imaging provides a valuable tool for monitoringdisease progression and response to therapy during treatment or drugtrials. The present invention is also useful for non-human applicationsof magnetic resonance, such as imaging of materials (e.g., plants orfood products) or animal models of disease at high field.

Further yet, an advantage of the present invention is that it provides ameans to shorten image acquisition times and/or increase spatialresolution for widely-used spin-echo-train magnetic resonance imagingtechniques. Such improvements will in turn make it feasible to obtainimages with certain valuable combinations of resolution and imagecontrast which have not been practical heretofore. In addition, thepresent invention permits spin-echo-train methods to be used forhigh-field imaging that would not otherwise meet the safety guidelinesestablished by the Food and Drug Administration for power deposition inhuman MRI.

Finally, another advantage of the present invention method and apparatusis that it explicitly considers the T1 and T2 relaxation times for thetissues of interest and thereby permits the desired image contrast to beincorporated into the tissue signal evolutions corresponding to the longecho train. Given the considerable role that spin-echo-train methodsalready play in MR imaging, the present invention methodology will be ofsignificant importance.

All US patents and US patent applications cited herein are incorporatedherein by reference in their entirety and for all purposes to the sameextent as if each individual patent or patent application wasspecifically and individually indicated to be incorporated by referencein its entirety for all purposes.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting of the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedherein.

We claim:
 1. A method for generating a spin echo pulse sequence foroperating a magnetic resonance imaging apparatus for imaging an objectthat permits at least one of lengthening usable echo-train duration,reducing power deposition and incorporating desired image contrast intothe tissue signal evolutions, said method comprising: a) providingcontrast-preparation, said contrast-preparation comprising generating atleast one of at least one radio-frequency pulse, at least onemagnetic-field gradient pulse, and at least one time delay, whereby saidcontrast preparation encodes the magnetization with at least one desiredimage contrast; b) calculating flip angles and phases of refocusingradio-frequency pulses that are applied in a data-acquisition step,wherein said calculation provides desired prescribed signal evolutionand desired overall signal level, said calculation comprises: i)selecting values of T1 and T2 relaxation times and selecting protondensity; ii) selecting a prescribed time course of the amplitudes andphases of the radio-frequency magnetic resonance signals that aregenerated by said refocusing radio-frequency pulses; and iii) selectingcharacteristics of said contrast-preparation step, said data-acquisitionstep and a magnetization-recovery step, with the exception of the flipangles and phases of the refocusing radio-frequency pulses that are tobe calculated; and c) providing said-data acquisition step based on aspin echo train acquisition, said data-acquisition step comprises: i) anexcitation radio-frequency pulse having a flip angle and phase; ii) atleast two refocusing radio-frequency pulses, each having a flip angleand phase as determined by said calculation step; and iii)magnetic-field gradient pulses that encode spatial information into atleast one of said radio-frequency magnetic resonance signals that followat least one of said refocusing radio-frequency pulses; d) providingmagnetization-recovery, said magnetization-recovery comprises a timedelay to allow magnetization to relax; and e) repeating steps (a)through (d) until a predetermined extent of spatial frequency space hasbeen sampled.
 2. The method of claim 1, wherein said calculation of theflip angles and phases is generated using an appropriate analytical orcomputer-based algorithm.
 3. The method of claim 1, wherein saidcalculation of the flip angles and phases is generated to account for,the effects of multiple applications of: said contrast-preparation, saiddata-acquisition and said magnetization-recovery steps, which arerequired to sample the desired extent of spatial-frequency space.
 4. Themethod of claim 1, wherein a two-dimensional plane of spatial-frequencyspace is sampled.
 5. The method of claim 1, wherein a three-dimensionalvolume of spatial-frequency space is sampled.
 6. The method of claim 1,wherein at least one of said contrast-preparation andmagnetization-recovery steps is omitted.
 7. The method of claim 1,wherein said calculation step is performed once before one of said firstcontrast-preparation step and said first data-acquisition step.
 8. Themethod of claim 1, wherein at least one of at least one saidcontrast-preparation step, at least one said data-acquisition step andat least one said magnetization-recovery step is initiated by a triggersignal to synchronizes the pulse sequence with at least one of at leastone external temporal event and at least one internal temporal event. 9.The method of claim 8, wherein said external and internal eventscomprise at least one of at least one voluntary action, at least oneinvoluntary action, at least one respiratory cycle and at least onecardiac cycle.
 10. The method of claim 1, wherein at least one of atleast one radio-frequency pulse and at least one magnetic-field gradientpulse is applied as part of at least one of at least one saidmagnetization-preparation step and at least one said data-acquisitionstep is for the purpose of stabilizing the response of at least one ofmagnetization related system and said apparatus related hardware system.11. The method of claim 1, wherein time duration varies betweenrepetitions for at least one of at least one said contrast-preparationstep, at least one said data-acquisition step and at least one saidmagnetization-recovery step.
 12. The method of claim 1, wherein the timeperiods between consecutive refocusing radio-frequency pulses appliedduring said data-acquisition steps are all of equal duration.
 13. Themethod of claim 1, wherein time periods between consecutive refocusingradio-frequency pulses applied during said data-acquisition steps varyin duration amongst pairs of refocusing radio-frequency pulses during atleast one said data-acquisition step.
 14. The method of claim 1 whereinall the radio-frequency pulses are at least one of non-spatiallyselective and non-chemically selective.
 15. The method of claim 1,wherein at least one of the radio-frequency pulses is at least one ofspatially selective in one of one, two and three dimensions, chemicallyselective, and adiabatic.
 16. The method of claim 1, wherein during eachsaid data-acquisition step, the phase difference between the phase forthe excitation radio-frequency pulse and the phases for all refocusingradio-frequency pulses is about 90 degrees.
 17. The method of claim 1,wherein during each data-acquisition step, the phase difference betweenthe phase for any refocusing radio-frequency pulse and the phase for theimmediately subsequent refocusing radio-frequency pulses is about 180degrees, and the phase difference between the phase for the excitationradio-frequency pulse and the phase for the first refocusing pulse isone of about 0 degrees and about 180 degrees.
 18. The method of claim17, wherein the flip angle for the excitation radio-frequency pulse isabout one-half of the flip angle for the first refocusingradio-frequency pulse.
 19. The method of claim 1, wherein thespatial-encoding magnetic-field gradient pulses applied during each saiddata-acquisition step are configured so as to collect data, followingeach of at least one of the refocusing radio-frequency pulses, for oneline in spatial-frequency space which is parallel to all other lines ofdata so collected, so as to collect the data using a magnetic resonanceimaging technique selected from the group consisting of rapidacquisition with relaxation enhancement (RARE), fast spin echo (FSE),and turbo spin echo (TSE or TurboSE).
 20. The method of claim 1, whereinthe spatial-encoding magnetic-field gradient pulses applied during eachsaid data-acquisition step are configured so as to collect data,following each of at least one of the refocusing radio-frequency pulses,for two or more lines in spatial-frequency space which are parallel toall other lines of data so collected, so as to collect the data using amagnetic resonance imaging technique selected from the group consistingof gradient and spin echo (GRASE) and turbo gradient spin echo (TGSE orTurboGSE).
 21. The method of claim 1, wherein the spatial-encodingmagnetic-field gradient pulses applied during each said data-acquisitionstep are configured so as to collect data, following each of at leastone of the refocusing radio-frequency pulses, for one or more lines inspatial-frequency space, each of which pass through one of a singlepoint in spatial-frequency space and a single line in spatial-frequencyspace, so as to collect the data using a magnetic resonance imagingtechnique selected from the group consisting of radial sampling orprojection-reconstruction sampling.
 22. The method of claim 21, whereinthe single point in spatial-frequency space is about zero spatialfrequency.
 23. The method of claim 21, wherein the single line inspatial-frequency space includes zero spatial frequency.
 24. The methodof claim 1, wherein the spatial-encoding magnetic-field gradient pulsesapplied during each said data-acquisition step are configured so as tocollect data, following each of at least one of the refocusingradio-frequency pulses, along a spiral trajectory in spatial-frequencyspace, each trajectory of which is contained in one of two dimensionsand three dimensions, and each trajectory of which passes through one ofa single point in spatial-frequency space and a single line inspatial-frequency space.
 25. The method of claim 24, wherein the singlepoint in spatial-frequency space is about zero spatial frequency. 26.The method of claim 24, wherein the single line in spatial-frequencyspace includes zero spatial frequency.
 27. The method of claim 1,wherein the spatial-encoding magnetic-field gradient pulses appliedduring at least one of said data-acquisition steps are configured tocollect sufficient spatial-frequency data to reconstruct at least twoimage sets, each of which exhibits contrast properties different fromthe other image sets.
 28. The method of claim 27, wherein at least someof the spatial-frequency data collected during at least one of saiddata-acquisition steps is used in the reconstruction of more than oneimage set, whereby the data is shared between image sets.
 29. The methodof claim 1, wherein the spatial-encoding magnetic-field gradient pulsesapplied during at least one of said data-acquisition steps areconfigured so that, for the echo following at least one of therefocusing radio-frequency pulses, at least one of the first moment, thesecond moment and the third moment corresponding to at least one of thespatial-encoding directions is approximately zero.
 30. The method ofclaim 1, wherein the spatial-encoding magnetic-field gradient pulsesapplied during at least one of said data-acquisition steps areconfigured so that, following at least one of the refocusingradio-frequency pulses, the zeroth moment measured over the time periodbetween said refocusing radio-frequency pulse and the immediatelyconsecutive refocusing radio-frequency pulse is approximately zero forat least one of the spatial-encoding directions.
 31. The method of claim1, wherein during all said data-acquisition steps the duration of alldata-sampling periods are equal.
 32. The method of claim 1, whereinduring at least one of said data-acquisition steps at least one of thedata-sampling periods has a duration that differs from the duration ofat least one other data-sampling period.
 33. The method of claim 1,wherein the spatial-encoding magnetic-field gradient pulses appliedduring said data-acquisition steps are configured so that the extent ofspatial-frequency space sampled along at least one of thespatial-encoding directions is not symmetric with respect to zerospatial frequency, whereby a larger extent of spatial-frequency space issampled to one side of zero spatial frequency as compared to theopposite side of zero spatial frequency.
 34. The method of claim 33wherein said spatial-frequency data is reconstructed using apartial-Fourier reconstruction algorithm.
 35. The method of claim 1,wherein during at least one of said data-acquisition steps the temporalorder in which spatial-frequency space data is collected for at leastone of the spatial-encoding directions is based on achieving at leastone of selected contrast properties in the image and selected propertiesof the corresponding point spread function.
 36. The method of claim 1,wherein during at least one of said data-acquisition steps the temporalorder in which spatial-frequency space data is collected is differentfrom that for at least one other data-acquisition step.
 37. The methodof claim 1, wherein during at least one of said data-acquisition stepsthe extent of spatial-frequency space data that is collected isdifferent from that for at least one other data-acquisition step. 38.The method of claim 1, wherein during at least one of saiddata-acquisition steps spatial encoding of the radio-frequency magneticresonance signal that follows at least one of the refocusingradio-frequency pulse is performed using only phase encoding so thatsaid signal is received by the radio-frequency transceiver in theabsence of any applied magnetic-field gradient pulses and hence containschemical-shift information.
 39. The method of claim 1, wherein at leastone navigator radio-frequency pulse is incorporated into the pulsesequence for the purpose of determining the displacement of a portion ofthe object.
 40. A magnetic resonance imaging apparatus generating a spinecho pulse sequence in order to operate the apparatus in imaging anobject that permits at least one of lengthening usable echo-trainduration, reducing power deposition and incorporating desired imagecontrast into the tissue signal evolutions, the apparatus comprising: amain magnet system generating a steady magnetic field; a gradient magnetsystem generating temporary gradient magnetic fields; a radio-frequencytransmitter system generating radio-frequency pulses; a radio-frequencyreceiver system receiving magnetic resonance signals; a reconstructionunit reconstructing an image of the object from the received magneticresonance signals; and a control unit generating signals controlling thegradient magnet system, the radio-frequency transmitter system, theradio-frequency receiver system, and the reconstruction unit, whereinthe control unit generates signals causing: a) providingcontrast-preparation, said contrast-preparation comprising generating atleast one of at least one radio-frequency pulse, at least onemagnetic-field gradient pulse, and at least one time delay, whereby saidcontrast preparation encodes the magnetization with at least one desiredimage contrast; b) calculating flip angles and phases of refocusingradio-frequency pulses that are applied in a data-acquisition step,wherein said calculation provides desired prescribed signal evolutionand desired over-all signal level, said calculation comprises: i)selecting values of T1 and T2 relaxation times and selecting protondensity; ii) selecting a prescribed time course of the amplitudes andphases of the radio-frequency magnetic resonance signals that aregenerated by said refocusing radio-frequency pulses; and iii) selectingcharacteristics of said contrast-preparation step, said data-acquisitionstep and a magnetization-recovery step, with the exception of the flipangles and phases of the refocusing radio-frequency pulses that are tobe calculated; and c) providing said-data acquisition step based on aspin echo train acquisition, said data-acquisition step comprises: i) anexcitation radio-frequency pulse having a flip angle and phase, ii) atleast two refocusing radio-frequency pulses, each having a flip angleand phase as determined by said calculation step, and iii)magnetic-field gradient pulses that encode spatial information into atleast one of said radio-frequency magnetic resonance signals that followat least one of said refocusing radio-frequency pulses; d) providingmagnetization-recovery, said magnetization-recovery comprises a timedelay to allow magnetization to relax; and e) repeating steps (a)through (d) until a predetermined extent of spatial frequency space hasbeen sampled.
 41. A magnetic resonance imaging apparatus generating aspin echo pulse sequence in order to operate the apparatus in imaging anobject that permits at least one of lengthening usable echo-trainduration, reducing power deposition and incorporating desired imagecontrast into the tissue signal evolutions, the apparatus comprising:main magnet means generating a steady magnetic field; gradient magnetmeans generating temporary gradient magnetic fields; radio-frequencytransmitter means generating radio-frequency pulses; radio-frequencyreceiver means receiving magnetic resonance signals;. reconstructionmeans reconstructing an image of the object from the received magneticresonance signals; and control means generating signals controlling thegradient magnet means, the radio-frequency transmitter means, theradio-frequency receiver means, and the reconstruction means, whereinthe control means generates signals causing: a) providingcontrast-preparation, said contrast-preparation comprising generating atleast one of at least one radio-frequency pulse, at least onemagnetic-field gradient pulse, and at least one time delay, whereby saidcontrast preparation encodes the magnetization with at least one desiredimage contrast; b) calculating flip angles and phases of refocusingradio-frequency pulses that are applied in a data-acquisition step,wherein said calculation provides desired prescribed signal evolutionand desired over-all signal level, said calculation comprises: i)selecting values of T1 and T2 relaxation times and selecting protondensity; ii) selecting a prescribed time course of the amplitudes andphases of the radio-frequency magnetic resonance signals that aregenerated by said refocusing radio-frequency pulses; and iii) selectingcharacteristics of said contrast-preparation step, said data-acquisitionstep and a magnetization-recovery step, with the exception of the flipangles and phases of the refocusing radio-frequency pulses that are tobe calculated; c) providing said-data acquisition step based on a spinecho train acquisition, said data-acquisition step comprises: i) anexcitation radio-frequency pulse having a flip angle and phase, ii) atleast two refocusing radio-frequency pulses, each having a flip angleand phase as determined by said calculation step, and iii)magnetic-field gradient pulses that encode spatial information into atleast one of said radio-frequency magnetic resonance signals that followat least one of said refocusing radio-frequency pulses; d) providingmagnetization-recovery, said magnetization-recovery comprises a timedelay to allow magnetization to relax; and e) repeating steps (a)through (d) until a predetermined extent of spatial frequency space hasbeen sampled.
 42. A computer readable media carrying encoded programinstructions for causing a programmable magnetic resonance imagingapparatus to perform the method of claim
 1. 43. A computer programprovided on a computer useable readable medium having computer programlogic enabling at least one processor in a magnetic resonance imagingapparatus to generate a spin echo pulse sequence that permits at leastone of lengthening usable echo-train duration, reducing power depositionand incorporating desired image contrast into the tissue signalevolutions, said computer program logic comprising: a) providingcontrast-preparation, said contrast-preparation comprising generating atleast one of at least one radio-frequency pulse, at least onemagnetic-field gradient pulse, and at least one time delay, whereby saidcontrast preparation encodes the magnetization with at least one desiredimage contrast; b) calculating flip angles and phases of refocusingradio-frequency pulses that are applied in a data-acquisition step,wherein said calculation provides desired prescribed signal evolutionand desired overall signal level, said calculation comprises: i)selecting values of T1 and T2 relaxation times and selecting protondensity; ii) selecting a prescribed time course of the amplitudes andphases of the radio-frequency magnetic resonance signals that aregenerated by said refocusing radio-frequency pulses; and iii) selectingcharacteristics of said contrast-preparation step, said data-acquisitionstep and a magnetization-recovery step, with the exception of the flipangles and phases of the refocusing radio-frequency pulses that are tobe calculated; and c) providing said-data acquisition step based on aspin echo train acquisition, said data-acquisition step comprises: i) anexcitation radio-frequency pulse having a flip angle and phase; ii) atleast two refocusing radio-frequency pulses, each having a flip angleand phase as determined by said calculation step; and iii)magnetic-field gradient pulses that encode spatial information into atleast one of said radio-frequency magnetic resonance signals that followat least one of said refocusing radio-frequency pulses; d) providingmagnetization-recovery, said magnetization-recovery comprises a timedelay to allow magnetization to relax; and e) repeating steps (a)through (d) until a predetermined extent of spatial frequency space hasbeen sampled.
 44. The method of claim 40, wherein at least one of saidcontrast-preparation and magnetization-recovery steps is omitted. 45.The method of claim 41, wherein at least one of saidcontrast-preparation and magnetization-recovery steps is omitted. 46.The method of claim 43, wherein at least one of saidcontrast-preparation and magnetization-recovery steps is omitted.
 47. Amethod of generating a T2-weighted fast-spin-echo or turbo-spin-echopulse sequence having refocusing radio-frequency pulses with varyingflip angles, said method comprising: generating, via a control unit, athree-dimensional T2-weighted fast-spin-echo or turbo-spin-echo pulsesequence used in operating a magnetic resonance imaging apparatus thatimages tissues of a human subject, the generated pulse sequence havingvarying flip angles that vary among a majority of refocusingradio-frequency pulses of an echo train by decreasing to a minimum valueand later increasing, and wherein the varying flip angles result in areduced power deposition compared to the power deposition that isobtained using refocusing radio-frequency pulses with constant flipangles of approximately 180 degrees; applying the pulse sequence to aradio-frequency transmitter coil of the magnetic resonance imagingapparatus to generate radio-frequency pulses in an examination zone thatincludes tissues of the human subject and receiving resulting magneticresonance signals from tissues of the human subject, using aradio-frequency receiver coil of the magnetic resonance imagingapparatus; and reconstructing magnetic resonance images from theresulting magnetic resonance signals from the tissues of the humansubject, wherein the reconstructed magnetic resonance images have aT2-weighted contrast, and wherein the T2-weighted contrast issubstantially the same as contrast in T2-weighted magnetic resonanceimages generated from a fast-spin-echo or turbo-spin-echo pulse sequenceusing refocusing radio-frequency pulses with constant flip angles ofapproximately 180 degrees.
 48. The method of claim 47, wherein the pulsesequence has a usable echo-train duration for spin-echo train imagingthat is lengthened substantially beyond that achievable with constant,low-flip-angle or pseudosteady-state approaches.
 49. The method of claim47, wherein the varying flip angles are calculated to provide aprescribed signal evolution.
 50. The method of claim 49, wherein theprescribed signal evolution is associated with T1 and T2 relaxationtimes that are arbitrarily chosen.
 51. The method of claim 50, whereinthe pulse sequence has a usable echo-train duration for spin-echo trainimaging that is lengthened substantially beyond that achievable withconstant, low-flip-angle or pseudosteady-state approaches.
 52. Amagnetic resonance imaging (MRI) apparatus that images tissues of ahuman subject and is configured to generate a T2-weighted fast-spin-echoor turbo-spin-echo pulse sequence having refocusing radio-frequencypulses with varying flip angles, the apparatus comprising: a main magnetsystem that generates a steady magnetic field; a gradient magnet systemthat generates temporary gradient magnetic fields; a radio-frequencytransmitter system that generates radio-frequency pulses; aradio-frequency receiver system that receives magnetic resonancesignals; a reconstruction unit that reconstructs magnetic resonanceimages of the subject from the received magnetic resonance signals fromthe tissues of the human subject; and a control unit that generatessignals controlling the gradient magnet system, the radio-frequencytransmitter system, the radio-frequency receiver system, and thereconstruction unit, wherein the control unit further provides signalsthat generate: a three-dimensional T2-weighted fast-spin-echo orturbo-spin-echo pulse sequence used in operating the MRI apparatus, thegenerated pulse sequence having varying flip angles that vary among amajority of refocusing radio-frequency pulses of an echo train bydecreasing to a minimum value and later increasing, and wherein thevarying flip angles result in a reduced power deposition compared to thepower deposition that is obtained using refocusing radio-frequencypulses with constant flip angles of approximately 180 degrees, whereinthe reconstructed magnetic resonance images have a T2-weighted contrast,and wherein the T2-weighted contrast is substantially the same ascontrast in T2-weighted magnetic resonance images generated from afast-spin-echo or turbo-spin-echo pulse sequence using refocusingradio-frequency pulses with constant flip angles of approximately 180degrees.
 53. The apparatus of claim 52, wherein the pulse sequence thatis generated has a usable echo-train duration for spin-echo trainimaging that is lengthened substantially beyond that achievable withconstant, low-flip-angle or pseudosteady-state approaches.
 54. Theapparatus of claim 52, wherein a control unit executes a softwareprogram that performs calculations and is configured to calculate thevarying flip angles to provide a prescribed signal evolution.
 55. Theapparatus of claim 54, wherein the prescribed signal evolution isassociated with T1 and T2 relaxation times that are arbitrarily chosen.56. The apparatus of claim 55, wherein the pulse sequence that isgenerated has a usable echo-train duration for spin-echo train imagingthat is lengthened substantially beyond that achievable with constant,low-flip-angle or pseudosteady-state approaches.
 57. A non-transitorycomputer readable medium having computer program logic that whenimplemented enables one or more processors in a magnetic resonanceimaging apparatus that images tissues of a human subject to generate aT2-weighted fast-spin-echo or turbo-spin-echo pulse sequence havingrefocusing radio-frequency pulses with varying flip angles, saidcomputer program logic comprising: logic for generating athree-dimensional T2-weighted fast-spin-echo or turbo-spin-echo pulsesequence used in operating the magnetic resonance imaging apparatus, thegenerated pulse sequence having varying flip angles that vary among amajority of refocusing radio-frequency pulses of an echo train bydecreasing to a minimum value and later increasing, and wherein thevarying flip angles result in a reduced power deposition compared to thepower deposition that is obtained using refocusing radio-frequencypulses with constant flip angles of approximately 180 degrees; and logicfor reconstructing magnetic resonance images from magnetic resonancesignals received from tissues of the human subject as a result ofapplying the generated pulse sequence, wherein the reconstructedmagnetic resonance images have a T2-weighted contrast, and wherein theT2-weighted contrast is substantially the same as contrast inT2-weighted magnetic resonance images generated from a fast-spin-echo orturbo-spin-echo pulse sequence using refocusing radio-frequency pulseswith constant flip angles of approximately 180 degrees.
 58. Thenon-transitory computer readable medium having computer program logic asdefined in claim 57, wherein the pulse sequence that is generated has ausable echo-train duration for spin-echo train imaging that islengthened substantially beyond that achievable with constant,low-flip-angle or pseudosteady-state approaches.
 59. The non-transitorycomputer readable medium having computer program logic as defined inclaim 57, wherein the computer program logic also calculates the varyingflip angles to provide a prescribed signal evolution.
 60. Thenon-transitory computer readable medium having computer program logic asdefined in claim 59, wherein the prescribed signal evolution isassociated with T1 and T2 relaxation times that are arbitrarily chosen.61. The non-transitory computer readable medium having computer programlogic as defined in claim 60, wherein the pulse sequence that isgenerated has a usable echo-train duration for spin-echo train imagingthat is lengthened substantially beyond that achievable with constant,low-flip-angle or pseudosteady-state approaches.